The present invention generally relates to electronic equipment, and more particularly to a system that utilizes passive eddy current sensors to sense rotating equipment, such as the arrival, position, and/or vibration of turbine blades and other moving objects.
Passive eddy current sensors and variable reluctance sensors have been employed in a wide variety of applications to sense the proximity and speed of rotating equipment, including blades (buckets) of gas turbines. Another application is to sense the positions of rotating blades within the fan, booster, compressor and turbine sections of a gas turbine engine to monitor the health of the engine. In particular, the output of a passive eddy current sensor (or other suitable position sensor) can be used to monitor blade vibrations and steady-state blade circumferential positions over the life of the engine. Changes in blade vibrations or blade static positions can indicate damage to the component and signal that an inspection may be required to prevent a catastrophic failure of an engine component.
Passive eddy current sensors typically contain one or more permanent magnets adjacent one or more ferromagnetic cores wound with a wire coil. The permanent magnet is typically formed of a high magnetic energy product material, notable examples of which include iron-rare earth metal alloys (for example, Nd—Fe—B) and samarium alloys (for example, Sm—Co). The core is typically formed of a magnetic steel, though other suitable magnetic materials including low carbon steels may be used depending on operating conditions. When used to monitor the vibration of blade tips, a passive eddy current sensor is mounted to maximize the electrical signal generated as each blades passes in proximity to the sensor. In particular, the sensor is oriented so that, in the absence of a blade, magnetic flux is directed through one end of the magnet and toward the rotor and its blades, then arcs back through space to the ferromagnetic core. When a blade passes through the magnetic field, eddy currents form in the blade material and the local magnetic field shifts, producing a voltage potential across the leads of the coil. Because engine casings are typically formed largely of titanium, nickel, and other nonferrous materials that exhibit low magnetic reluctance, the ends of the magnet and core are not required to be inserted entirely through the engine casing, but instead can be mounted in an external recess in the wall such that a portion of the wall separates the sensor from the hot gas path of the engine.
In modern gas turbine engines, the output of a passive eddy current sensor used to monitor blade vibration is delivered to the engine's FADEC (full authority digital engine control) through appropriate connectors and wiring. Passive eddy current sensors are susceptible to electromagnetic interference (EMI) noise due to the many turns of wire typically present and required in the construction of their cores, as well the long cable runs between the sensor and the engine FADEC. U.S. Pat. No. 3,932,813 to Gallant is an example of a probe design with multiple coils capable of addressing EMI noise encountered when attempting to measure the speed of turbomachinery. The Gallant sensor has an E-shaped core whose center leg is a magnet and whose outer legs are formed of a ferromagnetic material. The center magnetic establishes a symmetrical magnetic field through the two outer legs, each of which is wound with a wire coil. The coils are connected in series with a simple wire connection, with the result that EMI and other unwanted disturbances are subtracted from the output signal of the sensor.
The sensor taught by Gallant is disclosed as suitable for measuring the speed of a turbomachine, and not the position and vibrations of individual blades. Evaluations of passive eddy current sensors configured in accordance with Gallant have shown that the combined resistance and inductance of the wire and coils are too great for the sensor to have sufficient bandwidth to accurately sense the position and vibrations of individual airfoils. Such sensors also suffer from output wave shape limitations. Other examples of passive eddy current sensors with wire connections between coils for the purpose or having the effect of canceling noise include U.S. Pat. No. 4,967,153 to Langley, U.S. Pat. No. 5,373,234 to Kulczyk, and U.S. Pat. No. 6,483,293 to Chen. However each of these sensor designs suffers from decreased bandwidth and waveshape variations due to the combined resistance and inductance associated with having two coils wired in series.
More recent passive eddy current sensor designs specifically intended for blade detection are disclosed in U.S. Pat. Nos. 6,927,567 and 7,170,284 to Roeseler et al. Each of the disclosed sensors is a single-coil probe design intended or otherwise believed to improve signal bandwidth. However, neither appears to address the issue of operating in an EMI environment, and therefore these prior sensors do not appear to be capable of producing reliable measurements in a high EMI environment.
In view of the above, it would be desirable if a passive eddy current sensor were available that was capable of exhibiting the EMI resistance of multi-coil probe designs, while also capable of achieving the high bandwidth capability of single-coil probe designs, thereby providing the capability of sensing the position of gas turbine blades and other moving objects.